1. Field of the Invention
The present invention relates to an image signal processor and an electronic endoscope system which carries out an effective image process on image signals so that blood vessels included in an image photographed by an electronic endoscope, are clearly displayed.
2. Description of the Related Art
An electronic endoscope system, used for observing internal organs and making a diagnosis, is well known. The electronic endoscope system illuminates the internal organs, captures an image of the illuminated internal organ, and displays the captured image on a monitor. Capillaries exist at a shallow depth under an inner wall of an organ. Being able to observe capillaries contributes greatly to making a good diagnosis and identifying cancer in an organ. However, it has been difficult to clearly observe capillaries because the inner wall is reddish as are the capillaries, and the capillaries are minute.
Therefore, an electronic endoscope system, that clearly displays blood vessels, is proposed. The endoscope system extracts an area displaying the blood vessels from a whole image based on image signals generated by an imaging device. The endoscope system increases the contrast of the extracted area to a level higher than that of the other areas in the whole image. And then the blood vessel can be clearly displayed. However, it is difficult to extract an area having many blood vessels in the case where the internal organ is illuminated by usual white light. This is because the strength of reflected light at minute capillaries is less than that at other areas.
The relative strength of reflected light at capillaries can be increased by lighting up an inner wall with light having a short wavelength. However, the light having a short wavelength increases not only the relative strength of reflected light at a capillary, but also the relative strength at another organ and another blood vessel that exist in a littler deeper depth than the capillaries. Accordingly, it is difficult to instantly observe only capillaries.